Abstract
The search for high voltage cathodes for lithium-ion batteries has led to recent interest in the monoclinic Li 2 Fe(SO 4) 2 material which has a voltage of 3.83 V vs. lithium, the highest recorded for a fluorine-free iron-based compound. Here we investigate the defect, surface and lithium migration properties of the Li 2 M(SO 4) 2 (M ¼ Fe, Mn, Co) materials using combined atomistic modelling and density functional theory (DFT) techniques. All intrinsic defect types including Li/M antisite disorder are found to be of high energy, suggesting insignificant concentrations. Low activation energies are found for lithium migration along the a-axis channels giving rise to long-range 1D diffusion, which are supported by molecular dynamics (MD) simulations. For the crystal morphology a significant surface area is exposed to these 1D diffusion channels, which would allow facile Li insertion and extraction. Using DFT simulations we reproduce the high voltage of the Li 2 Fe(SO 4) 2 material in accord with electrochemical data and also examine local structural distortions on lithium extraction.